Optical waveguide type optical modulator and production method therefor

ABSTRACT

There is provided a high performance optical waveguide type optical modulator with excellent long term reliability, in which contamination of the buffer layer in a forming process of a signal field adjustment region on the buffer layer by a lift-off method or an etching method, is prevented and DC drift thus suppressed. The optical waveguide type optical modulator  10  comprises a substrate  11  having an electro-optic effect, optical waveguides  12  formed on the surface of this substrate  11,  a traveling-wave type signal electrode  13   a  and ground electrodes  13   b  which are provided on the substrate  11  and control a lightwave, and a buffer layer  14  provided between the electrodes  13  and the optical waveguides  12,  and furthermore, a dielectric layer  15  is provided on the entire surface of the buffer layer  14  on the side of the electrodes  13,  and a signal field adjustment region  16  which has a wider width than that of the traveling-wave type signal electrode  13   a  and is made of a material with a higher refractive index than that of the dielectric layer  15  is formed between the dielectric layer  15  and the traveling-wave type signal electrode  13   a.

TECHNICAL FIELD

The present invention relates to an optical waveguide type opticalmodulator suited to use in optical fiber communication systems and thelike, and a manufacturing method thereof.

BACKGROUND ART

Recently, optical waveguide type optical modulators 140 as shown in FIG.9 have become very common in optical fiber communication systems. Inthis optical waveguide type optical modulator 140, optical waveguides142 are formed on a substrate 141 which has an electro-optic effect, anda guided lightwave which travels in he optical waveguides 142 iscontrolled by a traveling-wave type signal electrode 143 a and groundedelectrodes 143 b. In the optical waveguide type optical modulator 140 ofthis example, normally, a buffer layer 144 made of an insulatingmaterial such as silicon oxide is formed on the substrate 141 to preventabsorption of the guided lightwave which travels in the opticalwaveguide 142 by the electrodes 143. Furthermore, a signal fieldadjustment region 145, which has wider width than that of thetraveling-wave type signal electrode 143 a, is formed between thetraveling-wave type signal electrode 143 a and the buffer layer 144,allowing the effective refractive index of the microwaves whichpropagate the electrodes 143 to be adjusted.

Such an optical waveguide type optical modulator 140 can be manufacturedusing the following method.

At first, after the optical waveguides 142 are formed by thermaldiffusion on the surface of the substrate 141 which is made of aferroelectric substance, the buffer layer 144 is formed on the substrate141 using methods such as vacuum deposition or sputtering. Subsequently,the signal field adjustment region 145 is formed at a predeterminedposition on this buffer layer 144. Then, the traveling-wave type signalelectrode 143 a is formed at a predetermined position on the signalfield adjustment region 145, and the ground electrodes 143 b are formedat predetermined positions on the buffer layer 144, at differentpositions from the signal field adjustment region 145.

In such a manufacturing process, the signal field adjustment region 145is normally formed using a lift-off method or an etching method.

In a lift-off method, at first, a photoresist is applied to the bufferlayer 144. Then after exposing a desired pattern onto the photoresistusing a photomask, this resist pattern is developed to make the maskedportions, excluding the predetermined position where the signal fieldadjustment region 145 is to be formed. Subsequently, a film of the metalor semiconductor or the like which forms the signal field adjustmentregion 145 is deposited thereon, and by removing the resist using aresist removal agent, the film on the resist is removed at the sametime, thereby forming the signal field adjustment region 145 at apredetermined position on the buffer layer 144.

An example of an etching method is a wet etching method employing aliquid etchant. In order to form the signal field adjustment region 145using a wet etching method, firstly, a film of the metal orsemiconductor or the like which forms the signal field adjustment region145 is deposited on the buffer layer 144, and a photoresist is appliedthereon. After exposing a desired pattern onto the photoresist using aphotomask, this resist pattern is developed, masking the predeterminedposition where the signal field adjustment region 145 is to be formed.Then, the exposed portions of the film are removed using an etchant ofmixed acid or the like. The remaining photoresist is then removed usinga resist removal agent, thereby forming the signal field adjustmentregion 145 at a predetermined position on the buffer layer 144.

However, when the signal field adjustment region is formed on the bufferlayer using a lift-off method, because the photoresist is applieddirectly to the buffer layer and hardened, the principal components andthe diluent which constitute the photoresist may penetrate the bufferlayer, causing contamination of the buffer layer. In particular, if thebuffer layer is silicon oxide, the surface of the buffer layer issometimes treated with vapor of an amine-based compound in order toimprove the adhesive strength of the photoresist to the silicon oxide,and in such a case, the buffer layer can also be contaminated by theamine-based compound.

Furthermore, when the resist pattern is developed, or when a metal orsemiconductor is deposited to form the signal field adjustment region,solutions such as resist developer or special chemical agents are usedin these processes. Consequently, there is a danger that the penetrationof the resist components into the buffer layer may be acceleratedbecause components from the resist can dissolve in these solutions.Moreover, these solutions may also be a source of contaminationthemselves.

Furthermore, when the signal field adjustment region is formed using awet etching method, there is a danger that the buffer layer iscontaminated by contacting the etchant or the resist removal agent.

If the buffer layer is contaminated, DC drift in the optical waveguidetype optical modulator is promoted by the presence of ions derived fromthe contaminants, thereby reducing the long-term reliability of theoptical waveguide type optical modulator. Moreover, in the case thatdegree of contamination being high, the insulating properties of thebuffer layer deteriorate greatly, allowing a portion of, or most of theelectric field applied from the electrodes to be leaked through thebuffer layer, which means that an electric field cannot be efficientlygenerated in the optical waveguide, and even though a signal fieldadjustment region is provided, the effect of the region may beinadequate.

Such contamination of the buffer layer has a great influence on theperformance of the optical waveguide type optical modulator. In order tosuppress this contamination, it is necessary to control the density andthe microstructure or the like of the buffer layer itself, and alsoprecisely control the conditions for the lift-off process and theetching process, but it is extremely difficult to control theseconditions.

On the other hand, FIG. 10 shows a cross-sectional view of anotherexample of a conventional optical waveguide type optical modulator. Thisexample is similar to that shown in FIG. 9, but differs in terms ofstructure.

This optical waveguide type optical modulator uses a ferroelectricsubstrate made of lithium niobate (LiNbO₃), which is the most common andpractical material for optical waveguide type optical modulators usingferroelectric substrates.

In FIG. 10, reference numeral 210 indicates a Z-cut ferroelectricsubstrate made of lithium niobate. The axis inducing the electro-opticeffect of this ferroelectric substrate 210 is in the direction of the Zaxis, which is the main optical axis (the crystallographic c axis), andas shown in FIG. 10, is aligned with a direction orthogonal to thesurface where the optical waveguides 202 are formed (termed as the “mainsurface” in this specification) in the ferroelectric substrate 210.

The optical waveguides 202, fabricated by thermal diffusion of Ti, areformed near the main surface of the ferroelectric substrate 210, and abuffer layer 203 made of SiO₂ is formed thereon. In addition, electrodes204 made of Au are formed on the buffer layer 203 so as to be parallelto the optical waveguides 202. A transition metal layer 205 consistingof a transition metal such as Ti, Cr, Ni or the like is provided betweenthe electrode 204 and the buffer layer 203.

Such an optical waveguide type optical modulator can be manufacturedusing a method in which, firstly, the optical waveguides 202 are formedon the main surface of the ferroelectric substrate 210 by means ofthermal diffusion, and then the buffer layer 203 is formed on the sideof the ferroelectric substrate 210 on which the optical waveguides 202are formed, using a method such as vacuum deposition or sputtering.Then, a transition metal film and an Au film are formed sequentially onthe entire upper surface of the buffer layer 203 by vacuum deposition. Athick Au layer accumulated on this Au film by an electroplating process,only within an electrode forming region, where the electrodes 204 are tobe formed, thereby forming the electrodes 204. Subsequently, the Au filmand the transition metal film remaining between the electrodes 204 isremoved by chemical etching, to obtain a transition metal layer 205.

However, because the buffer layer 203 of such an optical waveguide typeoptical modulator is exposed between the electrodes 204, the modulatorhas a disadvantage that an exposed surface 203 a of the buffer layer 203and the inside of the buffer layer 203 are easily contaminated bycontaminants such as K, Ti and Cr.

In particular, when vacuum deposition method is used to form the bufferlayer 203 with a low density so as to control the characteristics of theoptical waveguide type optical modulator, a problem that contaminantscan easily penetrate the buffer layer 203 via the exposed portions maybe occurred.

If the surface 203 a of the buffer layer 203 of the optical waveguidetype optical modulator and the inside of the buffer layer 203 arecontaminated, DC drift may occur. DC drift refers to a phenomenon thatthe presence of alkali ions such as K or Na and mobile ions like protoncauses the electric current leakage applied to the electrode 204 throughthe buffer layer 203, causing that the desired voltage (bias) cannot beapplied, which has a negative effect on the characteristics of theoptical waveguide type optical modulator.

In addition, if the contaminants in the buffer layer 203 reach to theinterface between the ferroelectric substrate 210 and the buffer layer203 as a result of thermal treatment performed during the mountingprocess of modulator chip or the like, the chemical bonds of the SiO₂buffer layer 203 can be broken by the contaminants, reducing the bondsbinding the ferroelectric substrate 210 comprising lithium niobate andthe buffer layer 203. As a result, it is expected that the bondingstrength between the ferroelectric substrate 210 and the buffer layer203 is weakened remarkably.

DISCLOSURE OF THE INVENTION

An optical waveguide type optical modulator according to a firstembodiment of the present invention comprises a substrate having anelectro-optic effect, optical waveguides formed on the surface of thissubstrate, a traveling-wave type signal electrode and ground electrodeswhich are provided on the substrate and control a guided lightwave, anda buffer layer provided between the electrodes and the opticalwaveguides, and furthermore, a dielectric layer is provided on theentire surface of the buffer layer on the side of the electrodes, and asignal field adjustment region which has wider width than that of thetraveling-wave type signal type electrode and is made of a material witha higher refractive index than that of the dielectric layer is formedbetween the dielectric layer and the traveling-wave type signalelectrode. In the optical waveguide type optical modulator describedabove, the signal field adjustment region may be made of silicon, andthe substrate may be made of lithium niobate, the buffer layer may bemade of silicon oxide, and the dielectric layer may be made of siliconnitride or silicon oxynitride. Furthermore, the thickness of thedielectric layer may be less than that of the thickness of the signalfield adjustment region.

A method of manufacturing this type of optical waveguide type opticalmodulator comprises a process (1) in which the dielectric layer isformed on the entire surface of the buffer layer, and a process (2) inwhich the signal field adjustment region is formed at a predeterminedposition on the dielectric layer. Furthermore, in this manufacturingmethod, the process (2) may be a process wherein the signal fieldadjustment region is formed at a predetermined position on thedielectric layer by applying a mask to those portions on the dielectriclayer excluding the predetermined position where the signal fieldadjustment region is to be formed, depositing a film of the materialwhich forms the signal field adjustment region, and then removing themask. In this case, the mask is a photoresist, and the removal of themask may be performed using a resist removal agent. Furthermore, theprocess (1) for forming the dielectric layer in this manufacturingmethod may be performed by a sputtering method.

As shown above, in the present invention, because the dielectric layeris formed on the entire surface of the buffer layer, when the signalfield adjustment region is formed by a lift-off method or an etchingmethod, resist components, the resist removal agent, and chemicaletchant or the like do not directly contact the buffer layer.Accordingly, an optical waveguide type optical modulator with excellentlong-term reliability can be provided in which contamination of thebuffer layer can be prevented, and DC drift caused by such contaminationcan be suppressed.

Furthermore, if the buffer layer is made of silicon oxide, thedielectric layer is made of silicon nitride or silicon oxynitride, andthe signal field adjustment region is made of silicon, then the opticalwaveguide type optical modulator has superior high frequency properties.And because the refractive index of the dielectric layer can be definedarbitrarily by varying the ratio of oxygen to nitrogen in the dielectriclayer, the high frequency properties are more widely adjustable. Inaddition, because the interfaces between these layers are stronglyjoined by covalent bonding via silicon (Si), an optical waveguide typeoptical modulator in which the films have excellent bonding strength canbe obtained.

An optical waveguide type optical modulator according to a secondembodiment of the present invention comprises a ferroelectric substratemade of a single crystal having an electro-optical effect and withoptical waveguides formed on a main surface thereof, and a buffer layerand electrodes provided on the main surface side of the ferroelectricsubstrate, wherein the axis of the ferroelectric substrate inducing anelectro-optical effect is orthogonal to the main surface of theferroelectric substrate, and an electrically insulating protective filmwith a thickness of 50 to 200 nm is provided on the upper surface of thebuffer layer at least in those regions where the electrodes are notformed, and on the side surfaces of the buffer layer parallel to thelightwave guiding direction. Here, in the optical waveguide type opticalmodulator of this embodiment, it is preferable that this protective filmis provided on the whole area of the upper surface of the buffer layerincluding the regions where the electrodes are formed, or that theprotective film provided on the upper surface of the buffer layer andthe protective film provided on the side surfaces of the buffer layerparallel to lightwave-guiding direction are made of the same materials.Furthermore, this protective film is preferably an amorphous film.

A method of manufacturing such an optical waveguide type opticalmodulator comprises a process for forming optical waveguides on thesurface of a ferroelectric substrate, which is made of a single crystalhaving an electro-optic effect, and the axis inducing an electro-opticaleffect is orthogonal to the main surface; a process for forming a bufferlayer on the side of the ferroelectric substrate on which the opticalwaveguides are formed; a process for forming an electrically insulatingprotective film with a thickness of 50 to 200 nm on the buffer layer, atleast in those regions excluding an electrode forming region; and aprocess for forming electrodes in the electrode forming region.

Furthermore, in the method of manufacturing the optical waveguide typeoptical modulator described above, the protective film is preferablyformed after performing heat treatment, which the ferroelectricsubstrate having the buffer layer is heated in a film depositionapparatus. The buffer layer in this optical waveguide type opticalmodulator sometimes adsorb moisture during the interval between thebuffer layer formation and the protective film formation, which has anegative effect on the stability of the operation of the opticalwaveguide type optical modulator. In particular, if the density of thebuffer layer is low, then the buffer layer adsorbs moisture from theatmosphere quite easily.

According to the method of manufacturing an optical waveguide typeoptical modulator described above, because the ferroelectric substratehaving the buffer layer undergoes heat treatment in the film depositionapparatus preceding the formation of the protective film even if thebuffer layer adsorbs moisture during the interval between the bufferlayer formation and the protective film formation, this moisture can beremoved by the heat treatment. Accordingly, an optical waveguide typeoptical modulator with excellent stability of operation can be obtained.

In this manner, in the optical waveguide type optical modulator of thepresent invention, because the protective film is provided on the bufferlayer, at least in the regions where the electrodes are not formed, andon the side surfaces of the buffer layer parallel to thelightwave-guiding direction, the surface or the buffer layer and theside surface of the buffer layer in the optical waveguide direction arenot exposed. Consequently, an optical waveguide type optical modulatoris obtained in which the surface of the buffer layer and the inside ofthe buffer layer are not easily contaminated.

Accordingly, leakage of the electric current applied to the electrodescaused by contaminants on the surface or inside of the buffer layer canbe prevented, and stability in the operation of the optical waveguidetype optical modulator can be ensured. Furthermore, even if a DC biassuperposing on high frequency voltage is applied to the electrodes, goodstability can still be achieved in this state. In other words, theoccurrence of DC drift can be prevented. In addition, because the bufferlayer is not easily contaminated, a reduction in the bonding strengthbetween the ferroelectric substrate and the buffer layer caused bycontamination of the buffer layer is hard to occur.

Furthermore, because the protective film is electrically insulating,leakage of the electric current applied to the electrodes can be morereliably prevented, and the operational stability of the opticalwaveguide type optical modulator can be ensured. Accordingly, theeffects achieved by preventing the occurrence of DC drift can be furtherenhanced. In addition, because the thickness of the protective layer isbetween 50 and 200 nm, the occurrence of DC drift can be effectivelyprevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an optical waveguide typeoptical modulator of a first embodiment.

FIG. 2 is a process diagram showing an example of a method of forming asignal field adjustment region in the optical waveguide type opticalmodulator shown in FIG. 1.

FIG. 3 is a process diagram showing another example of a method offorming a signal field adjustment region in the optical waveguide typeoptical modulator shown in FIG. 1.

FIG. 4 is a graph showing the relationship between the applied biasvoltage and the operation time of examples 1 and 2 and a comparativeexample 1.

FIG. 5 is a cross-sectional view showing an example of an opticalwaveguide type optical modulator of a second embodiment.

FIG. 6 is a cross-sectional view showing another example of an opticalwaveguide type optical modulator of the second embodiment.

FIG. 7 is a cross-sectional view showing yet another example of anoptical waveguide type optical modulator of the second embodiment.

FIG. 8 is a graph showing the relationship between the applied biasvoltage and the operation time of examples 3 through 5, a comparativeexample 2 and a conventional example.

FIG. 9 is a cross-sectional view showing a first configuration exampleof a conventional optical waveguide type optical modulator.

FIG. 10 is a cross-sectional view showing a second configuration exampleof a conventional optical waveguide type optical modulator.

BEST MODE FOR CARRYING OUT THE INVENTION

As follows is a description of preferred embodiments of an opticalwaveguide type optical modulator according to the present invention,with reference to the drawings. However, the present invention is notlimited to the following embodiments, and any kind of variation ormodification such as combinations of structural elements of differentembodiments may be made.

FIG. 1 is a cross-sectional view showing the structure of an opticalwaveguide type optical modulator 10 according to a first embodiment ofthe present invention, and the reference numeral 11 indicates asubstrate made of a ferroelectric substance such as lithium niobate.

Because it is relatively easy to grow the large crystals of lithiumniobate which are used as the single crystal substrates in the opticalwaveguide type optical modulators in the present application, integrateddevices having a large scale can be realized by using a ferroelectricsubstrate made of lithium niobate.

Furthermore, since single crystal of lithium niobate has high Curietemperature approximately up to 1000° C., there is a high degree offreedom with regard to temperature in the manufacturing process of theoptical waveguide type optical modulator.

Optical waveguides 12 fabricated by thermal diffusion of titanium areformed on the surface of this substrate 11, so that the lightwave isguided in the lengthwise direction of the optical waveguides 12.Electrodes 13 which control the guided lightwave are formed on thesubstrate 11 on which the optical waveguides 12 are formed, and in thisexample, a traveling-wave type signal electrode 13 a is provided in thecenter of the substrate 11, and ground electrodes 13 b are provided onboth sides thereof. Furthermore, a buffer layer 14 with a thickness of200 to 2000 nm is provided between the electrodes 13 and the opticalwaveguides 12 on the entire surface of the substrate 11, so that theguided lightwave which travels the optical waveguides 12 is not absorbedby the electrodes 13.

Furthermore, a dielectric layer 15 with a thickness of 10 to 200 nmwhich prevents contamination of the buffer layer 14 during themanufacturing process of the optical waveguide type optical modulator 10is provided on the entire surface of the buffer layer 14.

In addition, a signal field adjustment region 16 with a thickness of 10to 200 nm, which has wider width than that of the traveling-wave typesignal electrode 13 a and is made of a material with a higher refractiveindex than that of the dielectric layer 15, is provided between thedielectric layer 15 and the traveling-wave type signal electrode 13 a,allowing the effective refractive index of the microwaves whichpropagate the electrodes 13 to be adjusted.

The buffer layer 14 is usually formed from a substance with a smalldielectric constant, that is, a small refractive index. Forming thebuffer layer 14 from such a substance is desirable because the highfrequency properties (bandwidth) of the modulator can be extended. Anexample of a suitable material is silicon oxide, which is alsochemically stable.

For the same reasons as for the buffer layer 14, the dielectric layer 15is also preferably formed from a substance with a small refractiveindex, and examples of suitable materials include silicon nitride andsilicon oxynitride. Furthermore, the refractive index of the siliconoxynitride can be controlled arbitrary by adjusting the ratio ofnitrogen and oxygen from a value similar to that of silicon, through toa value similar to that of silicon oxide.

Furthermore, the signal field adjustment region 16 is formed from atypical metal or a semiconductor, as disclosed in Japanese UnexaminedPatent Application, First Publication No. Hei. 10-3065. Accordingly, therefractive index of the signal field adjustment region 16 is inevitablyhigher than that of the buffer layer 14 and the dielectric layer 15.

In this manner, a material with a low refractive index is selected toform the buffer layer 14 and dielectric layer 15, so as to suppressdegradation in the high frequency properties of the optical waveguidetype optical modulator 10. In addition, in order to get furtherextension of the high frequency properties, it is preferable that thedielectric layer 15 is made of a material with a lower refractive indexthan that of the buffer layer 14. If the dielectric layer 15 is made ofa material with a higher refractive index than that of the buffer layer14, then the thickness of the dielectric layer is preferably as thin aspossible, and preferably having the same or less than 100 nm.Furthermore, if the thickness of the dielectric layer 15 is the same orless than that of the signal field adjustment region 16, then the highfrequency properties of the optical waveguide type optical modulator 10are further improved.

If silicon oxide is used as the buffer layer 14, silicon nitride orsilicon oxynitride is used as the dielectric layer 15, and silicon isused as the signal field adjustment region 16, then the opticalwaveguide type optical modulator 10 has excellent high frequencyproperties, and since the refractive index of the dielectric layer 15can be defined arbitrarily, the degree of design freedom of the highfrequency properties is expanded. In addition, since the interfacesbetween these layers are strongly joined by covalent bonding via silicon(Si), the bonding strength between the films is favorably improved.

Silicon oxide (Si—O), which has a low refractive index, is suitable foruse as the buffer layer which directly contacts the optical waveguides.If the buffer layer is made of silicon oxide, the protective film can bemade of silicon (Si), silicon nitride (Si—N), silicon oxynitride(Si—O—N) or silicon oxide (Si—O) or the like, with favorable results.

If the dielectric layer 15 is provided on the entire surface of thebuffer layer 14 in this manner, then a high performance opticalwaveguide type optical modulator 10 in which contamination duringmanufacturing process of the buffer layer 14 is suppressed, resulting inhaving a suppressed DC drift property can be manufactured in the mannerdescribed below.

A method of manufacturing the optical waveguide type optical modulator10 according to the first embodiment shown in FIG. 1 is described belowwith reference to FIG. 2A through FIG. 2D. At first, the opticalwaveguides 12 fabricated by thermal diffusion of titanium are formed onthe surface of the substrate 11 which has an electro-optic effect. Thebuffer layer 14 is formed by a vacuum deposition method or the like onthe substrate 11 on which the optical waveguides 12 are formed. At thistime, in order to adequately oxidize the buffer layer 14, heat treatmentis performed under an oxidizing atmosphere at 500 to 700° C. forapproximately 5 to 10 hours. Then, by using a process (1), thedielectric layer 15 is formed on the entire surface of the buffer layer14, as shown in FIG. 2A. Because the dielectric layer 15 is provided inorder to prevent contamination of the buffer layer 14, it is preferablyas dense as possible, and is usually formed by a normal sputteringmethod.

Then by using a process (2), the signal field adjustment region 16, madeof a material with a higher refractive index than that of the dielectriclayer 15, is formed at a predetermined position on the dielectric layer15.

The formation of the signal field adjustment region 16 in the process(2) is performed using a lift-off method or an etching method or thelike, although a lift-off method is preferred because the process issimple, and the process conditions are easier to set compared with anetching method.

In the case of forming the signal field adjustment region 16 using alift-off method, at first, a photoresist 18 is spin coated on the entiresurface of the dielectric layer 15 and subsequently hardened, and thesection where the signal field adjustment region 16 is to be formed isthen exposed using a photomask.

By developing exposed photoresist, the photoresist within the sectionswhere the signal field adjustment region 16 is to be formed is removed,and the sections excluding the predetermined section where the signalfield adjustment region 16 to be formed are being masked state by thephotoresist 18 as shown in FIG. 2B.

Then, as shown in FIG. 2C, a film 16 a made of a material such assilicon, which forms the signal field adjustment region 16, is depositedon the entire surface of the dielectric layer 15 which is masked by thephotoresist 18, using a sputtering method or a vacuum deposition method.

Then by removing the mask using a resist removal agent, the film can beremoved along with the mask except at the predetermined position wherethe signal field adjustment region 16 is formed, resulting in the signalfield adjustment region 16 being formed only within this predeterminedposition, as shown in FIG. 2D.

A method of forming the signal field adjustment region 16 using a wetetching method is shown in FIG. 3A through FIG. 3D.

After forming the dielectric layer 15 on the buffer layer 14 as shown inFIG. 3A, a film 16 a of the metal or semiconductor which forms thesignal field adjustment region 16 is deposited on the dielectric layer15, and a photoresist 18 is spin-coated thereon. After exposure of thepattern for the signal field adjustment region 16 onto this photoresist18 using a photomask, the resist pattern is developed and thepredetermined position where the signal field adjustment region 16 is tobe formed is being a masked state by the photoresist 18, as shown inFIG. 3B. Then, only the exposed portions of the film 16 a are removedusing an chemical etchant such as a mixed acid or the like (FIG. 3C).Finally, by removing the remaining photoresist 18 using a resist removalagent, the signal field adjustment region 16 is formed only at thepredetermined position on the dielectric layer 15 as shown in FIG. 3D.

If the signal field adjustment region 16 is formed using a lift-offmethod in the manner described above, because the dielectric layer 15 isprovided on the buffer layer 14, the photoresist 18 is spin-coated andhardened on the dielectric layer 15, not on the buffer layer 14.Consequently, the principal resist components and the diluent whichconstitute the photoresist 18 do not penetrate the buffer layer 14, andcontamination of the buffer layer 14 can be prevented. Furthermore, inthe case that surface treatment using the vapor of an amine-basedcompound or the like is required so as to improve the adhesion of thephotoresist 18, provided the buffer layer 14 do not expose the surfacetreatment agent because the dielectric layer 15 is provided on thebuffer layer 14, and consequently contamination caused by this surfacetreatment can be suppressed.

In addition to suppressing the contamination caused by the photoresist18, contamination of the buffer layer 14 caused by solutions such asresist developer and resist removal agent can be suppressed when thesesolutions are employed for developing or removing the resist pattern ordepositing the metal or semiconductor which forms the signal fieldadjustment region 16. Furthermore, the penetration into the buffer layer14 of resist components which have dissolved in these solutions, canalso be suppressed.

And in the case that the signal field adjusting region 16 is formedusing a wet etching method, the buffer layer 14 is not contaminatedbecause the etching solution and resist removal agent do not directlycontact the buffer layer 14 if the dielectric layer 15 is provided.

In this manner, by providing the dielectric layer 15, contamination ofthe buffer layer 14 is prevented, and consequently, DC drift caused byions derived from such contaminants can be suppressed, allowing themanufacture of an optical waveguide type optical modulator 10 withexcellent long-term reliability. Furthermore, any reduction in thedielectric properties of the buffer layer 14 caused by contamination canalso be suppressed, and there is no danger that a portion of, or most ofthe electric field from the electrodes 13 dissipate through the bufferlayer 14, resulting in an electric field being efficiently applied tothe optical waveguides 12. Accordingly, the effects of providing thesignal field adjustment region 16 can be fully realized.

Furthermore, according to such a manufacturing method, because anoptical waveguide type optical modulator 10 with excellent reliabilitycan be manufactured by simply providing a dielectric layer 15 on thebuffer layer 14, it is not necessary to strictly control the processconditions of the lift-off process or the etching process.

After forming the signal field adjustment region 16 in this manner, thetraveling-wave type signal electrode 13 a is formed on the signal fieldadjustment region 16 by an Au electroplating method or the like, and theground electrodes 13 b are formed on the exposed dielectric layer 15exposing to both sides of the signal field adjustment region 16, therebycompleting the manufacture of the optical waveguide type opticalmodulator 10.

Such an optical waveguide type optical modulator 10 can be used as anoptical intensity modulator or a phase modulator, and can also be usedas an integrated modulator, by combining plural devices.

EXAMPLES

As follows is a detailed description of examples of the embodiments ofthe present invention.

Example 1

The optical waveguide type optical modulator 10 shown in FIG. 1 wasmanufactured in the following manner.

Optical waveguides 12 were formed on the surface of the ferroelectricsubstrate 11, which is made of lithium niobate, by performing diffusionprocessing of an optical waveguide pattern made of titanium having athickness of 90 nm, for 15 hours under an oxygen atmosphere and at atemperature of 1000° C. A silicon oxide (SiO₂) film with a thickness of1000 nm, which forms the buffer layer 14, was then deposited on thesubstrate using a vacuum deposition method. After performing heattreatment at 500° C. in a stream of oxygen, a dielectric layer 15, whichis made of silicon nitride and has a thickness of 100 nm, was formed onthe entire surface of the buffer layer 14 by a sputtering method.

Then, after a photoresist was spin-coated on the entire surface of thedielectric layer 15 and subsequently hardened, a signal field adjustmentregion 16 was exposed onto the photoresist using a photomask. Afterdevelopment and removal of the section of the photoresist where thesignal field adjustment region 16 is to be formed, a silicon film wasformed by a sputtering method. The resist was removed using a resistremoval agent, and the entire film except for the predetermined positionwhere the signal field adjustment region 16 is to be formed was alsoremoved together with the resist, thereby forming a signal fieldadjustment region 16 made of silicon having a thickness of 100 nm in thecenter of the substrate.

After the signal field adjustment region 16 was formed by means of anelectrolytic plating process and a traveling-wave type signal electrode13 a made of Au was formed on this signal field adjustment region 16,and ground electrodes 13 b, also made of Au, were formed on thedielectric layer 15, exposing both sides of the signal field adjustmentregion 16.

Example 2

The optical waveguide type optical modulator 10 shown in FIG. 1 wasmanufactured in the same manner as in the example 1, with the exceptionthat the dielectric layer 15 was formed of silicon oxynitride.

Comparative Example 1

The optical waveguide type optical modulator was manufactured in thesame manner as in the example 1, with the exception that a dielectriclayer was not formed.

Test Results 1

The stability of the optical waveguide type optical modulators obtainedin the examples 1 and 2 and the comparative example 1 were evaluatedusing the following method.

Namely, each optical waveguide type optical modulator was placed in athermostatic oven set to be 85° C., and an initial DC bias of 3.5 V wasapplied. The optical waveguide type optical modulator was then operatedfor 24 hours, and feedback control of the applied DC bias was performed,while checking the modulation state of the optical output signal usingan oscilloscope, so that the modulation state of the signal remained inthe same state as when the initial DC bias was applied, and changes inthe applied DC bias were recorded over the 24 hours. The results areshown in FIG. 4.

As shown in FIG. 4, in the optical waveguide type optical modulators ofthe present embodiment in which the dielectric layer was formed, theapplied DC bias shows small rise compared with the comparative example,showing that DC drift was suppressed. Accordingly, long-term operationalstability can be improved by using an optical waveguide type opticalmodulator of the present embodiment.

Next, an optical waveguide type optical modulator according to a secondembodiment of the present invention is described.

FIG. 5 is a cross-sectional view showing an example of an opticalwaveguide type optical modulator of the second embodiment. This opticalwaveguide type optical modulator also uses a ferroelectric substratemade of lithium niobate (LiNbO₃), which is the most common and practicalmaterial for optical waveguide type optical modulators employing aferroelectric substrate.

In FIG. 5, reference numeral 21 indicates a Z-cut ferroelectricsubstrate made of lithium niobate. The axis inducing the electro-opticeffect of this ferroelectric substrate 21 is in the direction of the Zaxis, which is the main optical axis (the crystallographic c axis), andis in a direction orthogonal to the main surface of the ferroelectricsubstrate 21 as shown in FIG. 5.

Optical waveguides 22 fabricated by thermal diffusion of Ti are formedon the main surface of the ferroelectric substrate 21. Here, the “mainsurface” refers to the surface of the ferroelectric substrate on whichthe optical waveguides are formed. The lightwave guiding direction ofthis optical waveguide type optical modulator is the lengthwisedirection of the optical waveguides 22. A buffer layer 23 is formed onthe optical waveguides 22, and a protective film 27 is formed on theentire upper surface 23 a of the buffer layer 23 as well as on the sidesurfaces 23 b of the buffer layer 23 in the direction parallel to theoptical waveguide. In addition, electrodes 24 made of Au are formed onthe protective film 27 so as to be parallel to the optical waveguides22. In FIG. 5, the electrode 24 positioned in the center is a signalelectrode, and the electrodes 24 positioned in both sides thereof areground electrodes. A transition metal layer 25 made from a transitionmetal such as Ti, Cr, or Ni or the like is provided between theelectrodes 24 and the protective film 27.

In this optical waveguide type optical modulator, the buffer layer 23 ismade of SiO₂ with a low refractive index.

Furthermore, the protective film 27 is an electrically insulatingamorphous film having a thickness from 50 to 200 nm, which is also madeof SiO₂, the same material as the buffer layer 23.

In order to manufacture such an optical waveguide type opticalmodulator, at first, the optical waveguides 22 are formed on the surfaceof the ferroelectric substrate 21 by thermal diffusion, and the bufferlayer 23 is subsequently formed by a vacuum deposition method on theside of the ferroelectric substrate 21 on which the optical waveguides22 are formed. At this time, in order to adequately oxidize the bufferlayer 23, heat treatment (annealing) is performed under an oxidizingatmosphere at 500 to 700° C. for approximately 5 to 10 hours. Next, theferroelectric substrate 21 having the buffer layer 23 is placed in afilm deposition apparatus for forming the protective film 27, and theprotective film 27 is formed on the entire surface 23 a of the bufferlayer 23 using a sputtering method. At this time, before forming theprotective film 27, heat treatment may be performed with an object ofremoving the moisture (H₂O, —OH) in the buffer layer 23, and increasingthe hardness of the protective film 27. Subsequently, a transition metalfilm and an Au film are formed sequentially by a vacuum deposition orsputtering method. In addition, a thick Au layer is accumulated on thisAu film by an electroplating process, only within an electrode formingregion, where the electrodes 24 are to be formed, thereby forming theelectrodes 24. Subsequently, the Au film and the transition metal filmremaining between the electrodes 24 is removed by chemical etching,thereby forming a transition metal layer 25 only beneath the electrodes24.

Next, the ferroelectric substrate 21 is cut into a chip shape, and aprotective film 27 is formed on the side surfaces 23 b of the bufferlayer 23. To form the protective film 27 on the side surfaces 23 b ofthe buffer layer 23, the entire side surface of the chip other than theside surfaces 23 b of the buffer layer 23 is covered using a resist orthe like, and a protective film 27 is formed using the same sputteringmethod or the like used for the protective film 27 formed on the bufferlayer 23.

In the manufacturing method described above, the protective film 27 maybe formed either duration of heat treatment or the period after the heattreatment has been completed. In this case, the condition of heattreatment may be set, foe example, to be a temperature of 100 to 300°C., a processing time of 1 to 20 hours, and a degree of vacuum of 1×10⁻⁵to 1×10⁻¹ Pa.

In such an optical waveguide type optical modulator according to thesecond embodiment, since the protective film 27 is provided on theentire upper surface 23 a and the side surfaces 23 b of the buffer layer23, the surface 23 a and the side surfaces 23 b of the buffer layer 23are not exposed, and contamination of the surface 23 a of the bufferlayer 23 or the inside of the buffer layer 23 is hard to occur.

In the optical waveguide type optical modulator described above, theprotective film is preferably provided on the entire upper surface ofthe buffer layer, including the area where the electrodes are formed. Insuch an optical waveguide type optical modulator, the entire surface ofthe buffer layer is covered by the protective film, and consequentlycontamination of the buffer layer during the manufacturing processsucceeding after the protective layer formation can be prevented, andthe buffer layer is more effectively protected from the penetration ofcontaminants.

In addition, because the protective film is formed on the entire surfaceof the buffer layer, the protective layer is easier to form than in acase that the protective film is formed only on a part of the surface ofthe buffer layer.

Furthermore, the protective film provided on the surface of the bufferlayer and the protective film provided on the side surfaces of thebuffer layer in the direction parallel to the optical waveguide arepreferably made of the same material.

Using such a configuration of an optical waveguide type opticalmodulator, it is possible to improve the chemical bonding strengthbetween the protective layer provided on the buffer layer and theprotective layer provided on the side surfaces of the buffer layer inthe direction parallel to the optical waveguide. Furthermore, becausethe thermal expansion characteristics of the protective layer providedon the buffer layer and the protective layer provided on the sidesurfaces of the buffer layer are the same, the adhesion at the interfacebetween these layers is also thermally stable. These factors allow theprotective layer to have enhanced resistant effect for the buffer layercontamination and to have greater stability.

In addition, because the protective film 27 is electrically insulating,leakage of the electric current applied to the electrodes 24 can beprevented, and stable operation of the optical waveguide type opticalmodulator can be ensured. Accordingly, the occurrence of DC drift can beprevented. Furthermore, since the buffer layer 23 is not easilycontaminated, a reduction in the bonding strength between theferroelectric substrate and the buffer layer caused by contamination ofthe buffer layer does not easily occur. In this case “electricallyinsulating” means a resistance for the direct current being greater than20 MΩ, and preferably greater than 50 MΩ.

Furthermore, because the buffer layer 23 is not easily contaminated,even if the density of the buffer layer 23 is low, problems caused bycontamination of the buffer layer is hard to occur. Consequently, thebuffer layer 23 can be formed using a vacuum deposition method, whichforms a buffer layer 23 with low density. Furthermore, it is possible toselect the density of the buffer layer 23 and the method of forming thebuffer layer 23 according to requirement.

In addition, because the thickness of the protective film 27 is between50 and 200 nm, the occurrence of DC drift can be effectively prevented.

It is not favorable for the thickness of the protective layer to be lessthan 50 nm, since the effect of preventing contamination of the surfaceand the inside of the buffer layer is insufficient, and the electriccurrent applied to the electrodes becomes easy to leak, meaning theoccurrence of DC drift cannot be sufficiently prevented. On the otherhand, it is not favorable for the thickness of the protective layer toexceed 200 nm, since the protective film having different propertiesfrom the buffer layer occupies a large proportion of the thickness ofthe buffer layer (approximately 1 μm) being adjusted and optimized itsdensity or the like to suppress DC drift, the effects of the bufferlayer are weakened remarkably resulting in a deterioration in the effectfor preventing the occurrence of DC drift.

Furthermore, because the protective film 27 is provided between thebuffer layer 23 and the transition metal layer 25 provided beneath theelectrodes 24, and the entire upper surface 23 a of the buffer layer 23is covered by the protective film 27, contamination of the buffer layer23 during the manufacturing process following the formation of theprotective film 27 can be prevented.

In other words, contamination of the buffer layer 23 by such processesas the chemical etching of the Au film and the transition metal filmperformed during the process of forming the transition metal layer 25and the electrodes 24 on the protective film 27 can be prevented.

Furthermore, because the protective film 27 is formed between the bufferlayer 23 and the transition metal layer 25, the buffer layer 23 and thetransition metal layer 25 do not contact each other, embrittlement ofthe transition metal layer 25 derived from its oxidation caused byabsorbed moisture by the buffer layer 23 can be prevented. As a result,weakening of the bonding strength between the buffer layer 23 and thetransition metal layer 25 causing embrittlement of the latter can beprevented, ensuring the reliability of the optical waveguide typeoptical modulator.

In addition, because the protective film 27 is formed on the entiresurface 23 a of the buffer layer 23, the protective film 27 is formedeasier compared with the case that the protective film 27 is formed on aportion of the surface 23 a of the buffer layer 23.

Moreover, since the protective film 27 is made of SiO₂, the physicalcharacteristics of the protective film 27 and the buffer layer 23, suchas the coefficient of thermal expansion and the like, are the same,resulting in the bonding strength between the protective film 27 and thebuffer layer 23 to be excellent.

In addition, because the ferroelectric substrate 21 is made of lithiumniobate, which is relatively easy to grow into large crystals, it ispossible to realize large scale integrated devices.

Furthermore, since single crystal of lithium niobate has high Curietemperature, at approximately up to 1000° C., there is a high degree offreedom with regard to temperature in the manufacturing process of theoptical waveguide type optical modulator.

Moreover, because the transition metal layer 25 is provided between theelectrodes 24 and the protective film 27, the bonding strength betweenthe electrodes 24 and the protective film 27 can be improved.

In other words, the transition metal layer 25 has a function of formingan alloy (solid solution) or an intermetallic compound at the interfacewith the electrodes 24, fabricated by chemically inactive Au at theinterface with the protective film 27, thereby functioning as anadhesive which joins the electrodes 24 and the protective film 27.Accordingly, the bonding strength between the electrodes 24 and theprotective film 27 can be improved.

Furthermore, because the protective film 27 is formed using a sputteringmethod in the manufacturing method described above, the protective film27 can be formed easily.

In addition, because the entire protective film 27 is made of the samematerial, the protective film 27 can be formed easily with fewmanufacturing steps.

Because the protective film is formed after the ferroelectric substrate21 having the buffer layer 23 undergoes heat treatment in the filmdeposition apparatus preceding the formation of the protective film 27,even if the buffer layer adsorbs moisture during the interval betweenthe buffer layer 23 formation and the protective film 27 formation, thismoisture can be removed by the heat treatment. Accordingly, an opticalwaveguide type optical modulator with excellent stability of operationcan be obtained.

In the optical waveguide type optical modulator of the presentinvention, it is preferable that the protective film 27 is formed on theentire upper surface 23 a and the side surfaces 23 b of the buffer layer23 as shown in FIG. 5, although the protective film is needed to provideat least on the sections of the buffer layer 23 where the electrodes 24are not formed.

For example, as shown in FIG. 6, the protective layer 27 may be providedon the surfaces of the buffer layer 23 where the electrodes 24 are notformed, and between the buffer layer 23 and a part of the electrodes 24,and on the side surfaces 23 b of the buffer layer 23. Alternatively, asshown in FIG. 7, a protective film 29 may be provided on the surfaces ofthe buffer layer 23 where the electrodes 24 are not formed, and on theside surfaces 23 b of the buffer layer 23.

Furthermore, in the optical waveguide type optical modulator of thepresent invention, as shown in the examples described above, it ispreferable that a substrate made of lithium niobate is used as theferroelectric substrate, although the ferroelectric substrate is notlimited to lithium niobate, and, for example, a substrate made oflithium tantalate could also be used.

In the optical waveguide type optical modulator of the presentinvention, the transition metal layer 25 may be provided between theelectrodes 24 and the protective film 27 as described in the examplesabove, although the transition metal layer 25 is not be necessarily toprovided at all, and there are no particular restrictions relating tothe transition metal layer 25.

In addition, in the optical waveguide type optical modulator of thepresent invention, the optical waveguides 22 can be formed by diffusingTi as described in the examples above, but the dopant to form theoptical waveguides are not restricted to this material.

Furthermore, in order to propagate high frequency electric signals tothe electrodes 24, it is preferable that the buffer layer 23 in theoptical waveguide type optical modulator of the present invention ismade of SiO₂ which has a low dielectric constant, and that theelectrodes 24 are made of Au which has sufficiently low electricalresistance, as described in the examples above, but the materials usedto form the buffer layer 23 and the electrodes 24 are not restricted tothe materials described above.

Moreover, in the optical waveguide type optical modulator of the presentinvention, the protective film may be made of SiO₂ as described in theexamples above, but any electrically insulating material can be used,and there are no particular restrictions.

For example, the protective film may be made of silicon (Si) or siliconoxynitride or the like. If silicon is used as the protective film, itcan be formed by methods such as RF sputtering, using a pure silicontarget and Ar gas as the sputtering gas. At this time, if thetemperature of the ferroelectric substrate is controlled to beapproximately 250° C., then an excellent Si film with few defect isobtained, which has extremely high electrical resistance, and issubstantially an insulator. Because a protective film made of silicon(Si) does not contain oxygen, the transition metal layer formed on theprotective layer does not deteriorate by its oxidation. Accordingly,there is no danger of oxidation of the transition metal layer causingembrittlement of the transition metal layer and weakening of the bondingstrength between the buffer layer and the transition metal layer, whichimproves the reliability of the optical waveguide type opticalmodulator.

Furthermore, in the optical waveguide type optical modulator of thepresent invention, the entire protective film may be made of the samematerial, as described in the examples above, but the protective filmprovided on the buffer layer and the protective film provided on theside surfaces of the buffer layer may be made of different materials,for example, and there are no particular restrictions to the materials.

In the method of manufacturing the optical waveguide type opticalmodulator of the present invention, the protective film 27 is preferablyformed after performing heat treatment of the ferroelectric substrate 21having the buffer layer 23 in a deposition apparatus, as described inthe examples above, but this heat treatment need not necessarily to beperformed.

Furthermore, in the method of manufacturing the optical waveguide typeoptical modulator of the present invention, the buffer layer 23 may beformed by a vacuum deposition method, as described in the examplesabove, but the method to form the buffer layer is not limited to vacuumdeposition, and the buffer layer 23 may also be formed using ahigh-energy film deposition method such as a sputtering method, forexample, to provided a buffer layer with high density.

In the method of manufacturing the optical waveguide type opticalmodulator of the present invention, there is some cases that thesurfaces on the buffer layer 23 where the electrodes 24 are not formedare not completely covered by the protective film, due to the deviationoccurring in the manufacturing process. However, even if the protectivefilm is not formed on a portion of the surface of the buffer layer 23where the electrodes 24 are not formed, the objects of the presentinvention can still be achieved when the protective film is formed onthe most part of the buffer layer 23 on which the electrodes 24 are notformed.

The optical waveguide type optical modulator according to the secondembodiment of the present invention is described in detail below usingexamples (see FIG. 5).

Example 3

Optical waveguides 22 were formed on the surface of a ferroelectricsubstrate 21 made of Z-cut lithium niobate by performing a diffusionprocessing of an optical waveguide pattern made of Ti having a thicknessof 90 nm, for 20 hours under an oxygen atmosphere and at a temperatureof 1000° C. A SiO₂ film with a thickness of 1 μm, which forms the bufferlayer 23, was deposited on the substrate using a vacuum depositionmethod. After performing heat treatment at 600° C. for 5 hours in astream of oxygen, an upper protective film 27 made of SiO₂ having athickness of 50 nm was formed on the entire surface of the buffer layer23 by employing RF sputtering.

A transition metal layer made of Ti having a thickness of 50 nm, and anAu film having a thickness of 50 nm were formed sequentially on theprotective film 27, using a vacuum deposition method within the samefilm deposition apparatus. A resist pattern was formed on the Au filmusing photolithographic techniques, and a thick Au layer was accumulatedon this Au film by an electroplating process, only within the portionswhere the resist pattern was not formed, namely the portions where theAu film was exposed, thereby forming the electrodes 24. The resist maskwas removed using an organic solvent, and the Au film and the transitionmetal film remaining between the electrodes 24 were removed by etchingusing solution of iodine and potassium iodide and a mixed solution ofammonia and hydrogen peroxide, respectively. Then, the ferroelectricsubstrate 21 was cut into a chip shape. After the cutting, the sidesurface protective film 27 was formed from SiO₂ having a thickness of 50nm by employing RF sputtering on the side surfaces 23 b of the bufferlayer 23. The optical waveguide type optical modulator shown in FIG. 5was obtained in this manner.

Example 4

Using the same ferroelectric substrate 21 as in the example 3, theoptical waveguide 22 and the buffer layer 23 were formed in the samemanner as in the example 3, and a protective film 27 with a thickness of200 nm was formed on the upper surface of the buffer layer 23 in thesame manner as in the example 3.

The transition metal layer 25 and the electrodes 24 were formed on theprotective film 27 in the same manner as in example 3, and after cuttingthe substrate into a chip shape as in the example 3, the side surfaceprotective film 27 with a thickness of 200 nm was formed on the sidesurfaces 23 b of the buffer layer 23, thereby obtaining the opticalwaveguide type optical modulator shown in FIG. 5.

Example 5

Using the same ferroelectric substrate 21 as in the example 3, theoptical waveguide 22 and the buffer layer 23 were formed in the samemanner as in the example 3, and the resulting substrate was placed inthe film deposition apparatus in which the protective film 27 is formed,and heat treatment was performed at 250° C. for one hour under a degreeof vacuum of 2×10⁻⁵ Pa. Subsequently, while maintaining the temperatureat 250° C., Ar gas was introduced in the chamber as the sputtering gasuntil its interior pressure to be 0.2 Pa, and RF sputtering wasperformed using a pure silicon target with no dopant, thereby formingthe upper protective film 27 with a thickness of 100 nm on the entiresurface of the buffer layer 23.

The transition metal layer 25 and the electrodes 24 were formed on thisprotective film 27 in the same manner as in the example 3. Theferroelectric substrate 21 was cut into a chip shape, and the sidesurface protective film 27 with a thickness of 100 nm was formed on theside surfaces 23 b of the buffer layer 23 using the same method employedto form the protective film 27 on the surface of the buffer layer 23,thereby obtaining the optical waveguide type optical modulator shown inFIG. 5. In this optical waveguide type optical modulator, theinterelectrode resistance between the signal electrode and the groundelectrode was greater than 30 MΩ.

Conventional Example

The optical waveguide 22 and the buffer layer 23 were formed on the sameferroelectric substrate 21 as in the example 3, and the transition metallayer 25 and the electrodes 24 were formed on this buffer layer 23 inthe same manner as in Example 2, thereby obtaining the optical waveguidetype optical modulator shown in FIG. 10.

Comparative Example 2

The optical waveguides 22, and the buffer layer 23 which has a thicknessof 100 nm, were formed in the same manner as in the example 3, using thesame ferroelectric substrate 21 as in the example 3, and the upperprotective film 27 with a thickness of 900 nm was formed on the entiresurface of the buffer layer 23 in the same manner as in the example 3.

Subsequently, the transition metal layer 25 and the electrodes 24 wereformed on this upper protective film 27 in the same manner as in theexample 3, thereby obtaining the optical waveguide type opticalmodulator shown in FIG. 5.

Test Results 2

The following tests were performed to examine the stability of theoptical waveguide type optical modulators obtained in the examples 3 to5, the conventional example, and the comparative example 2.

Namely, each of the optical waveguide type optical modulators obtainedin the examples 3 to 5, the conventional example, and the comparativeexample 2 were placed in a thermostatic oven set to be 85° C., and aninitial DC bias of 3.5 V was applied. The optical waveguide type opticalmodulator was operated for 60 hours, and feedback control of the appliedDC bias was performed, while checking the modulation state of theoptical output signal using an oscilloscope, so that the modulationstate of the signal remained in the same state as the initial DC biaswas applied, and changes in the applied DC bias were recorded.

The results are shown in FIG. 8.

As shown in FIG. 8, in the conventional example of the secondembodiment, having no protective film 27, the applied DC bias risesremarkably with time. On the other hand, in the examples 3 to 5, therise in DC bias is small compared to the conventional example.Furthermore, in the comparative example 2 in which the thickness of theprotective film 27 exceeds the desirable range, the rise in DC bias islarge compared to the examples 3 to 5.

Because smaller variation in applied DC bias means smaller DC drift andexcellent stability in the optical signal output from the opticalwaveguide type optical modulator, it is apparent that stability in theoperation of the optical waveguide type optical modulator can beimproved by providing the protective film 27.

Furthermore, it is also apparent that it is preferable for the thicknessof the protective film 27 to be the same or less than 200 nm.

INDUSTRIAL APPLICABILITY

The present invention relates to an optical waveguide type opticalmodulator using a ferroelectric crystal as a substrate, and amanufacturing method thereof. The optical waveguide type opticalmodulator of the present invention is ideally suited to use in opticalfiber communication systems and the like.

1. An optical waveguide type optical modulator comprising a substratehaving an electro-optic effect, optical waveguides formed on the surfaceof this substrate, a traveling-wave type signal electrode and groundelectrodes which are provided on the substrate and control a guidedlightwave, and a buffer layer provided between the electrodes and theoptical waveguides, and furthermore, a dielectric layer is provided onthe entire surface of the buffer layer on the side of the electrodes,wherein a signal field adjustment region which has a wider width thanthat the traveling-wave type signal electrode and is made of a materialwith a higher refractive index than that of the dielectric layer isformed between the dielectric layer and the traveling-wave type signalelectrode.
 2. An optical waveguide type optical modulator according toclaim 1, wherein the signal field adjustment region is made of silicon.3. An optical waveguide type optical modulator according to claim 1,wherein the substrate is made of lithium niobate, the buffer layer ismade of silicon oxide, and the dielectric layer is made of siliconnitride or silicon oxynitride.
 4. An optical waveguide type opticalmodulator according to claim 1, wherein, the thickness of the dielectriclayer is the same or less than that of the signal field adjustmentregion.
 5. An optical waveguide type optical modulator comprising aferroelectric substrate made of a single crystal having anelectro-optical effect and with optical waveguides formed on a mainsurface thereof, and a buffer layer and electrodes provided on the mainsurface side of said ferroelectric substrate, wherein the axis of saidferroelectric substrate inducing an electro-optical effect is orthogonalto said main surface of said ferroelectric substrate, and anelectrically insulating protective film with a thickness of 50 to 200 nmis provided on the upper surface of said buffer layer at least in thoseregions where said electrodes are not formed, and on the side surfacesof said buffer layer parallel to the guided lightwave direction.
 6. Anoptical waveguide type optical modulator according to claim 5, whereinsaid protective film is provided on the whole area of the upper surfaceof said buffer layer including the regions where said electrodes.
 7. Anoptical waveguide type optical modulator according to claim 5, whereinthe protective film provided on the upper surface of said buffer layerand the protective film provided on the side surfaces on said bufferlayer parallel to the lightwave-guiding direction are made of the samematerials.
 8. An optical waveguide type optical modulator according toclaim 5, wherein said protective film is an amorphous film.